Web design experts have described the 2012 London Olympic Games ticket website as "a mixture of the good, the bad and the ugly", criticising the lack of venue and seating information and the insistence on paying with Visa.

As the ticketing website went live, London organisers said that pleas for prospective purchasers to take their time and not all rush at once appeared to have borne fruit, reporting sustained interest throughout the day.

User experience agency Foolproof praised the ballot system, which gives users 42 days to apply for the 6.6m tickets available, but said that by creating a separate pared-down ticketing site organisers had made the process more complicated.

Its managing partner, Tom Wood, said the lengthy application window was a "very sensible move" but added: "The separation into two sites has diluted the user experience as well as making the application process more complicated and less intuitive than it needed to be. The Visa exclusivity aspect – especially the way it is communicated – seems to us a major mis-step."

The report by the agency, the largest of its type in Europe, said that "the excitement and emotion surrounding the games is not amplified by the ticketing site".

It also said there was not enough information on either site. "Main site information is patchy and particularly unhelpful for people with concerns about attending with children, older people or disabled spectators."

But organisers, who faced embarrassment when the official countdown clock launched with much fanfare on Monday night broke down, argue the simple ticketing site was designed to be as robust as possible and avoid technical issues.

The London Organising Committee of the Olympic Games chairman, Lord Coe, has argued that it is not possible to give detailed information on where each seat will be due to outstanding issues around broadcasting positions.

Visa promised to work on a solution that would allow those with cards that expire before August to submit their applications. Organisers had warned in advance literature and on the website that they would not be valid but Visa said it was working on a solution. Coe has had to defend Visa, an Olympic sponsor for 25 years, for being the only card that can be used to pay for tickets online. Cheques are accepted, but only through the post.

Colin Grannell, an executive vice-president for Visa Europe, also said that the company would look into providing non-Visa holders with a one-off account number to enable them to apply if they did not want a card.

"We've been promoting for ages the fact you need a Visa card to buy a ticket. If push comes to shove we'll find a way of giving people a one-off account number for that transaction," he said. "I don't think we'll have many of those because most people have got one, but we'll find a technical solution if required."